This invention relates to a flat sealing gasket made of a soft material, particularly a cylinder head gasket for internal combustion engines and is of the type which has an impregnated, non-woven mat which may be metal-reinforced and which has overlays on the sealing faces on one or both sides of the mat The polymerized impregnating agent which fills the pores of the non-woven mat to an extent of 60 to 90% by volume serves essentially for increasing the cross-sectional density and the strength of the mat.
The overlays serve for increasing the sealing pressure. Such overlays are profiled, raised annular parts which are made of elastically deformable material and which surround the gasket openings for the combustion chambers and/or for the fluid ports. In the areas where the overlays are provided, an improved seal is obtained because of the increased sealing pressure and further because the overlays elastically conform to the sealing faces. Similarly, annular, cross-sectionally approximately U-shaped frames may be inserted about the sealing edges of the gasket openings. Such frames, in addition to protecting the edges of the openings, also increase the sealing pressure and thus the sealing effect by virtue of their raised, profiled configuration.
If, upon installing the cylinder head gasket between the engine block and the cylinder head, only a relatively small tightening force is applied which clamps the two engine components to one another, only the profiled overlays are placed under stress, while the sealing mat itself is loaded less, so that it performs essentially only a carrier function. Such non-uniform stresses may lead, during engine service, to the destruction of the overlays and to heat-caused deformations in the running faces of the cylinders. In case higher tightening forces are applied, the elastic overlays and the corresponding soft material zones of the mat are, during engine operation, stressed beyond their elastic limit which results in a flow of the soft material and the overlays, whereupon permanent plastic deformations or their destruction occurs, and thus the desired increase in the sealing pressure will not be obtained.
During the operation of the internal combustion engine, particularly during engine start, a rapid heating and non-uniform heat distribution occur, particularly in the vertical direction from the combustion chambers. Such non-uniform heat distribution leads, similarly to a bimetal effect, to thermal misalignment (deformations) of the surfaces to be sealed. To provide for a compensation, the cylinder head is clamped to the cylinder block with non-uniformly distributed tightening force, so that, as a result, the gasket is exposed to non-uniform sealing pressure forces. The sealing pressure forces which are increased in certain areas, counteract the thermal deformations of the sealing faces and risks of a deformation, particularly in the sealing faces of the cylinder head are avoided.
According to German Patent No. 1,074,341 the non-uniform sealing pressure force affecting the gasket is obtained by deformation characteristics which are non-uniformly distributed over the gasket. This is achieved by the fact that the gasket has different thicknesses in different areas and different capacity to receive the impregnating agent in different areas or the impregnating agent is polymerized to a different extent in the different areas. In other constructions, the gaskets have depressions in order to concentrate the pressure on predetermined zones of the sealing face. Nevertheless, the thermal deformations of the sealing faces of the cylinder head are in most cases of such an extent that in order to effect a compensation, the gasket has to be stressed in those zones with extremely high pressure forces which, even if the gasket is constructed according to German Patent No. 1,074,341, stress the gasket in certain zones beyond the flow limit of the soft material and thus a permanent plastic deformation of the soft material results and the desired bias is not obtained.